Actually, it's not sold out, they're just messing with you. Apparently only Ticketmaster's tickets are sold out. You can still get them directly through House of Blues (or really, LiveNation). You have to sign up as a member (
http://www.hob.com/membership), but otherwise I don't think there's anything special you have to do.
I only know this because I signed up as a member a long time ago, and a couple weeks ago I got an email from HoB saying "These shows are sold out, but you can still get tickets here!!!", and Nightwish was one of those shows. Funny thing was, I checked Ticketmaster after that, and they still had tickets available. It was only a day or two later that the Ticketmaster tickets were all gone. I then got my ticket through hob.com, and it worked just fine (got it in the mail a couple days ago). Added bonus: it was a whole 40 cents cheaper than the TM version!
I think this must be part of LiveNation's move to get into the ticketing business and steal market share from Ticketmaster. So they give Ticketmaster a set of tickets to sell, but still hang onto a bunch for themselves. The silly thing is that they forgot to promote the fact that they have tickets to sell! Even if you go directly through LiveNation.com, the link for Nightwish tickets passes you over to ticketmaster.com. I guess they must still must be "beta testing" their ticket-selling operation or something. So it will be interesting to see how "sold out" the show really is at the end.
In exchange for this valuable bit of information, my only request is that the rest of you help keep Michael from standing/dancing near me during the show!
Neil